This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application entitled Gain Flattened Optical Fiber Amplifier earlier filed in the Korean Industrial Property Office on 14 Jan. 1999, and there duly assigned Serial No. 99-848 by that Office.
1. Field of the Invention
The present invention relates to a gain flattened optical fiber amplifier, and more particularly to a gain flattened optical fiber amplifier provided with a reflection filter.
2. Description of the Prior Art
Generally, gain flattened optical fiber amplifiers are configured to have a gain flattened with respect to the intensities of an input signal light and a pumping light within a specific range. However, conventional gain flattened optical fiber amplifiers have a problem in that it is difficult to maintain a desired gain flatness when the condition of the input signal light or pumping light varies. In particular, where a plurality of optical fiber amplifiers are connected together in a multistage fashion, an input signal light may vary in condition in accordance with a variation in the loss characteristics among the amplifiers, thereby resulting in increased errors during the transmission of signal lights.
Generally, the intensity of an input signal light is detected to vary the intensity of a pumping light in accordance with the detected intensity of the input signal light, thereby compensating for a variation in gain. To this end, two pumping light sources are typically used.
FIG. 1 illustrates the configuration of a conventional optical fiber amplifier including two pumping light sources. As shown in FIG. 1, the conventional optical fiber amplifier includes a first isolator 100, a first pumping light source 102, a first wavelength division multiplexer (WDM) 104, an erbium-doped optical fiber (EDF) 106, a second pumping light source 108, a second WDM 110, and a second isolator 112.
An input signal light is applied to the first WDM 104 after passing through the first isolator 100. In the first WDM 104, the input signal light is combined with a pumping light generated from the first pumping light source 102 arranged upstream from the EDF 106. The resultant light emerging from the first WDM 104 is incident to the EDF 106.
A pumping light, which is generated from the second pumping light source 108 arranged downstream from the EDF 106, is incident to the EDF 106 via the second WDM 110. The forward and reverse pumping lights incident to the EDF 106 serve to excite erbium ions existing in the EDF 106 from a ground state. In the EDF 106, the input signal light is amplified by virtue of lights emitted in a stimulated fashion from the excited erbium ions. Each of the first and second isolators 100 and 112 serves to prevent an amplified spontaneous emission (ASE) light reflected from an optical element such as an input/output connector from being incident again to the EDF 106 to degrade an amplification efficiency for the input signal light.
Such an optical fiber amplifier is configured to adjust the ratio of population inversion at the upstream stage of the EDF 106 in accordance with a variation in the intensity of the forward pumping light while adjusting the ratio of population inversion at the downstream stage of the EDF 106 in accordance with a variation in the intensity of the reverse pumping light.
However, there is a problem in that it is impossible to freely adjust the ratio of population inversion at each stage of the EDF 106 because the intensity of the associated pumping light has an influence on the entire stage of the EDF 106.
Known prior art techniques for providing a flattened gain are disclosed by the following U.S. patents, incorporated by reference herein: U.S. Pat. No. 5,271,024 to David R. Huber entitled Optical Fiber Amplifier And Laser With Flattened Gain Slope; U.S. Pat. No. 5,253,104 to Jean-Marc P. Delavaux entitled Balanced Optical Amplifier; U.S. Pat. No. 5,239,607 to Valeria L. da Silva et al. entitled Optical Fiber Amplifier With Flattened Gain; and U.S. Pat. No. 5,436,760 to Yukinobu Nakabayashi entitled Optical Fiber Amplifier With Gain Equalizing Circuit.
It is an object of the invention to provide a gain flattened optical fiber amplifier including a pumping light separating unit arranged between two EDFs and adapted to separate forward and reverse pumping lights from each other, thereby compensating for a variation in gain in accordance with a variation in the intensity of an input signal light.
In accordance with the present invention, this object is accomplished by providing an optical fiber amplifier comprising: a first amplifying unit for amplifying an input signal light using a first pumping light to obtain a predetermined gain; a second amplifying unit for re-amplifying the signal light amplified by the first amplifying unit, using a second pumping light, to obtain the same gain as the predetermined gain; and a pumping light separating unit for allowing the amplified signal light outputted from the first amplifying unit to enter the second amplifying unit while preventing the first pumping light from reaching the second amplifying unit and preventing the second pumping light from reaching the first amplifying unit, whereby the amplified signal light entering the second amplifying unit is allowed to be amplified only by the second pumping light.